J. Chem. SOC., Faraday Trans. I, 1984,80, 3307-3314 Structural Study of Aerosol-OT-stabilised Microemulsions of Glycerol Dispersed in n-Heptane BY PAUL D. I. FLETCHER, MOHAMED F. GALAL AND BRIAN H. ROBINSON* Chemical Laboratory, University of Kent, Canterbury, Kent CT2 7NH Received 17th February, 1984 Thermodynamically stable Aerosol-OT-stabilised dispersions of glycerol in n-heptane (microemulsions) have been studied using dynamic light scattering and viscometry. Up to five moles of glycerol can be dispersed per mole of aerosol-OT in n-heptane. The resulting solutions consist of discrete spherical droplets of glycerol stabilised by the surfactant. Droplet size is independent of temperature and depends primarily on the mole ratio (R) of glycerol to AOT according to hydrodynamic radius/nm = 1.7( f 0.2) + 0.88( f 0.15) R.The apparent interfacial area occupied per AOT molecule is ca. 20% less in the glycerol dispersion than in the corresponding water dispersion. Attractive interactions between the droplets increase as the microemulsion phase stability limit is approached. Microemulsions have attracted much recent interest, from both theoretical (thermo- dynamics, particle interactions) and practical (potential use as novel reaction media) viewpoints. Microemulsions of water dispersed in heptane using Aerosol-OT as stabiliser are known to consist of a thermodynamically stable, transparent dispersion of discrete water droplets with a surfactant layer in a continuous oil phase. In this paper we describe dynamic light-scattering and viscosity measurements on solutions containing glycerol dispersed in n-heptane in the presence of sodium bis(2-ethylhexyl) sulphosuccinate (AOT).The corresponding dispersed water system has already been extensively investigated, as for example ref. (1)-(3), and hence it is possible to compare the structural properties of the two systems. Two important structural features of the microemulsion systems involving water and glycerol may be considered. First, for the dispersed water system a simple geometrical calculation [eqn (5), vide infra] involving the molar volume of water and the surface area per AOT molecule at the interface provides a reasonable model for the prediction of the equilibrium size of the aggregates. A further test of this simple calculation is provided by glycerol systems since the molar volume of the dispersed phase is considerably different. Secondly, for the dispersed water systems interparticle attractive interactions are known to be present, particularly as the phase stability limit is appr~ached.~ The nature of the attractive interaction is still uncertain but it may be related to surfactant mobility in the interfacial r e g i ~ n .~ It is of interest, therefore, to determine these interactions when AOT is interfacially bound to a high-viscosity dispersed phase such as glycerol. The amount of glycerol that may be solubilised in n-heptane solutions of AOT as a function of temperature has been determined. The structures of the droplets formed have been determined as functions of composition and temperature using dynamic light-scattering and viscosity methods.33073308 MICROEMULSIONS OF GLYCEROL IN n-HEPTANE EXPERIMENTAL The solvent n-heptane, obtained from Fisons, was distilled from sodium metal, stored over type 4A molecular sieve and filtered prior to use. AOT was obtained from Sigma and used without purification. Many samples of AOT contain an impurity with a pK, z 5 which is thought to be present as a result of incomplete esterification during manufacture or partial hydrolysis on storage.6 The batch used for these experiments contained negligible amounts of this impurity, as determined by a titration procedure. Glycerol was obtained from Fisons. Microemulsions were prepared by weighing quantities of glycerol into a volumetric flask, adding AOT solutions in heptane and making up to the mark with n-heptane.Gentle manual shaking produced clear solutions in a few minutes. Dynamic light-scattering measurements were performed using a goniometer constructed in this laboratory in combination with a Spectraphysics model 168 2 W argon-ion laser (operating at 488 nm), a Malvern K7025 correlator and an EM1 9863 photomultiplier tube in a Malvern RR 102 housing system. Sample solutions were filtered through Millipore 0.22 pm filters directly into Hellma fluorescence cells, which were mounted in a transparent dish of toluene. Precision thermostatting (to f 0.1 “C) was achieved by circulating the toluene through an immersion coil in a Haake thermostat. Data which contained signal perturbations caused by the presence of dust in the sample were discriminated against and rejected using computer control of the instrument.Data were collected in ‘burst’ experiments of one or two seconds duration. If the total count collected was outside preset discriminator levels, the data from that ‘burst’ were rejected. The data were analysed according to the method of cumulants.’ The limiting values (as delay time-0) of the first and second derivatives of the semi-logarithmic plot of the intensity autocorrelation functions against time were taken as the first two moments of the cumulants expansion. Data were accepted when good agreement was obtained between values of the baseline calculated from total counts and that measured at long delay times. The baseline agreement was taken as good when the measured difference was found to be < 1% of the autocorrelation amplitude.Refractive indices were determined using a thermostatted Abbe refractometer. Viscosity measurements were made with an Ubbelohde viscometer. Densities were measured using a standard pyknometer. RESULTS AND DISCUSSION MICROEMULSION STABILITY Fig. 1 shows the amount of glycerol that may be solubilised in AOT solutions of n-heptane as a function of temperature. The results are expressed as plots of the molar ratio of glycerol to AOT ( R ) against temperature. The area under the curve shows the clear microemulsion region. Fig. 1 shows the results for 0.1 mol dm-3 AOT but the stability region is independent of AOT concentration in the range 0.02- 0.2 mol dm-3. As the temperature is increased the solution turbidity increases sharply in the region of the phase limit.Further increase in temperature leads to separation of a glycerol-rich phase which sediments. The line shown in fig. 1 is the point at which the solution turbidity sharply increases and it was measured by visual inspection. Ca. 5 mol glycerol can be solubilised per mol AOT at 0 O C , this amount decreasing with increasing temperature. Aggregates with R values of less than two are stable over a very wide temperature range. Primary solvation of the AOT headgroup and the sodium counterion probably requires 2-3 mol glycerol. In the corresponding water microemulsion system, 1 0 w - R ~ ~ ~ (i.e. [H,O]/[AOT] 3 10) structures are also stable over a wide temperature rangel and this RHIO value marks the transition from a hydrated ‘inverse micelle ’ to a microemulsion droplet containing water with properties similar to bulk water.8 This corresponding point would appear to be R = 2 in the glycerol system. The microemulsion map shows no low-temperature microemulsion phase limitP. D.1. FLETCHER, M. F. GALAL AND B. H. ROBINSON 3 309 5 4 R 3 2 1 0 0 20 40 60 80 T/OC Fig. 1. Stability map for the AOT +glycerol + heptane microemulsion system. AOT concen- tration, 0.1 mol dm+. within the range 0-90 "C. This is in contrast to the corresponding water system and suggests that the factors controlling this low-temperature phase separation are altered. There is some evidence to suggest that, in the water system, this process may be associated with desorption of AOT molecules from the oil/water interface.*q DYNAMIC LIGHT-SCATTERING RESULTS From measurements of the intensity autocorrelation function (g(2)) as a function of delay time (t), a value for the measured collective diffusion coefficient (D) and the corresponding correlation length ( I ) were obtained : D = z/2K2 (1) 1 = kT/67tvD (2) where K = (4nn/A) sin 812, q is the solvent viscosity and z is the limiting decay rate given by [ - d In (g(2))/dt] limit as t -+ 0.It was checked experimentally that the measured correlation lengths were inde- pendent of scattering angle (30 < 8/* < 150) for high-R-value dispersions at 10 and 30 "C. Note that critical behaviour of the solution results in the correlation lengths exhibiting an angular dependence.1° The normalised variance of the autocorrelation function was in the range l-lO% for low volume fractions of dispersed phase.At high volume fractions the variance increased and the presence of two exponentials was detected. This type of behaviour has been observed previously and is explained by the appearance of a self-diffusion process (in addition to the collective diffusion process) at high volume fractions.ll In this study only the collective diffusion coefficient is discussed.3310 MICROEMULSIONS OF GLYCEROL IN n-HEPTANE P 10 30 50 T/OC Fig. 2. Correlation length as a function of temperature. AOT concentration 0.1 mol dm-3; R values (a) 0.986, (b) 1.95, (c) 3.08 and (d) 4.26. Fig. 2 shows values of the correlation lengths obtained for various R-value microemulsions as a function of temperature.Correlation lengths increase both with increasing R and with increasing temperature. The measured correlation length, in general, contains a contribution from interparticle interactions and can be equated with the particle hydrodynamic radius only for the case of a spherical particle in the limit of infinite dilution. In order to obtain values for the hydrodynamic radii, data were obtained at different AOTconcentrations but at constant R value and temperature. The data were analysed to yield values of rH (the hydrodynamic radius) and a. a provides a quantitative measure of inter-particle interactions and is defined by D = Do(l +a+) (3) (for small 4) where Do is the infinite-dilution limiting value of D and 4 is the volume fraction of AOT and glycerol in the dispersion.Note that there is an important assumption contained in the above procedure which is that the composition of the aggregates does not change as the system is diluted. Two possible situations may arise. First, if glycerol were to be soluble in the absence of AOT to a significant extent in the heptane oil phase, then the droplets might be expected to shrink upon dilution. This could then offer an explanation for the observed decrease in correlation length with dilution of the particles. However, this is extremely unlikely since the solubility of glycerol in heptane was measured to be < 0.7 mmol dm-3 over the temperature range 20-100 OC.12 This upper limit is insignificant when compared with the lowest glycerol concentration of 20 mmol dm-3 employed in this work.Secondly, AOT may desorb to some significant extent from the glycerol/heptane interface as the AOT concentration is decreased. This effect would cause an increase inP. D. I. FLETCHER, M. F. GALAL AND B. H. ROBINSON 331 1 01 I I I I 0 1 2 3 4 R Fig. 3. Hydrodynamic radius as a function of R. 0, 10 "C; 0, 30 "C. the size of the droplets at low concentrations, but this is not observed experimentally. Also, as will be shown presently, the values obtained for the hydrodynamic radii as a function of R imply that the AOT does not desorb to any large extent. Fig. 3 shows derived values of hydrodynamic radii plotted as a function of R for two temperatures (10 and 30 "C). For both temperatures the data are well described by r,/nm = 1.7( f 0.2) + 0.88( f 0.15)R.(4) The value of the intercept is in good agreement with other measurements of the hydrodynamic radius of 'empty' inverse micelles of AOT.'? l3 The value of the slope of fig. 3 may be compared with the corresponding water system by the following calculation. For a spherical microemulsion droplet containing a core of phase s (volume per molecule = V,) and containing nAOT surfactant molecules which present an interfacial area of aAOT per AOT molecule, then, assuming all AOT is interfacially bound particle surface area = 4nr2 = ~AOTLZAOT particle core volume = 4nr3 = nAOT RV, J where r is the radius of the central droplet core Combining the two equations yields 3R Vgl y cerol r = ~ A O T (not the overall particle for the core radius as a function of R, where molecule (0.121 nm3 calculated from the bulk density).The model also assumes a monodisperse collection of droplets. The slope of fig. 3 may be equated with (3Glycerol/aAOT) to yield an apparent value for CIAOT. Values of 0.41 f0.07 and 0.51 k0.08 nm2 may be calculated in this way for the glycerol and water [data from ref. (2)] systems, respectively. At face value, the figures imply that either the AOT cfesorbs from the glycerol interface more than from the water interface (by up to 20%) is the volume of a single radius). ( 5 ) glycerol3312 15 10 -ff 5 0 MICROEMULSIONS OF GLYCEROL IN n-HEPTANE 0 1 2 3 4 Fig. 4. Interaction coefficient a as a function of R. 0, 30 "C; 0, 10 "C. R or that the AOT is more closely packed at the glycerol interface.The model of the particle system used in these calculations is very crude and ignores, for example, any possible polydispersity in the system. For this reason it is felt that the similarity of the two systems is more significant than the relatively minor difference revealed by the calculation. Fig. 4 shows the variation of a with R at 10 and 30 "C. For hard spheres with purely repulsive interactions a is thought to be ca. 1.S4 whilst it is predicted to be negative in the case of attractive interactions. It can be seen that a becomes negative as the microemulsion phase limit is approached by increasing either R or the temperature. This result is still valid even if it is assumed that the particles have expanded by 20% on dilution. It is therefore concluded that the observed increase in correlation length with temperature is caused by increasingly attractive interactions between the particles and not by an increase in size of the particles.The value of a for the corresponding water microemulsion system is - 2.1 f 0.4 for all RHZO values (1 1-50) at 25 oC.2 This corresponds approximately with the low-R limiting value observed in this work. Small-angle neutron-scattering data have been obtained for the water-droplet system in the region of the phase-stability limit. It was concluded that the droplet size does not change much but critical scattering behaviour, which is a manifestation of attractive interactions, was 0bserved.~9 lo, l5 VISCOSITY MEASUREMENTS The viscosity of dispersions is sensitive in general to particle shape and interparticle interactions.The method is relatively insensitive to particle size.16 It is, therefore, a useful complementary technique to dynamic light scattering for this type of system. Fig. 5 shows plots of qsp/C against C for a glycerol microemulsion (R = 3.00 at 10 and 30 "C). The data were fitted (at low C) toP. D. I. FLETCHER, M. F. GALAL AND B. H. ROBINSON 3313 .. 4 c W Fig. 0 1 I I I 0 0.05 0.1 0.15 C/g ~ r n - ~ 5. Reduced viscosity plots. R = 3.00. 0, 30 "C; x , 10 "C. where Cis the concentration (g ~ m - ~ ) of the dispersed phase, qsp is the specific viscosity [ = (v -qsolvent)/qsolvent], kH is the Huggins coefficient and [q] is the intrinsic viscosity. It can be seen that both plots have the same intercept ([q]) but that the 30 "C data show a significantly higher slope (kH[q12). The intrinsic viscosity is dependent on both particle shape and solvation as (7) [~ll = 4Oparticle + dU,olvent) where v are the partial specific volumes of the subscripted species, 6 is the weight of solvent associated with 1 g of particles and v is a shape parameter, which is 2.5 for spheres.Partial specific volumes were calculated from measured density data [ i i ~ h ~ = 1.07 0.04 (temperature/"C) g cmP3; o $ ~ ~ ~ ~ ~ ~ = 1.3 1 k 0.05 g ~ r n - ~ (inde- pendent of temperature)]. The measured values of [q] are 2.8 k0.3 and 2.7 k0.3 cm3 8-l at 10 and 30 "C, respectively. If it is assumed that 6 = 0, then the corresponding values of v are 3.4 f 0.4 and 3.0 f 0.4. These values imply axial ratios for the particles of ca.2-3.15 If, however, v is assumed to have a value of 2.5 (corresponding to a spherical shape) then 6 is calculated to be 1&20%. Since the particle must be solvated to some extent, it is concluded that the particles must be close to spherical in shape. Values of k , are found to be 3.8 f 0.9 and 6.4 f 1.5 at 10 and 30 "C, respectively. Theoretical estimates for the Huggins coefficient in the case of hard spheres range from 0.7-0.8.17 The experimental values clearly deviate from the hard-sphere value, the larger deviation being found closer to the phase-separation limit. The conclusion from the viscosity data is that the shape of the particles is constant with increasing temperatures and close to spherical. However, interdroplet interactions clearly increase with temperature.These results are totally consistent with the picture emerging from the dynamic light-scattering data.33 14 MICROEMULSIONS OF GLYCEROL IN n-HEPTANE To summarise, AOT-stabilised glycerol dispersions in a heptane continuous phase consist of discrete droplets which are close to spherical in shape. 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